1. Field of the Invention
This invention relates to a heavy load pneumatic radial tire used in trucks, busses, and the like. In particular, it relates to a tire having substantially circumferentially continuous ribs on opposite sides of the tire tread and being provided with a tread pattern formed so as to make the continuity of the respective ribs on the opposite sides asymmetric to thereby improve running performance at high-speed running on improved roads.
2. Prior Art
In general, the tires of the type for high-speed running on improved roads employs a tread sectioned into a plurality of circumferential ribs separated by a plurality of wide main grooves extending in the circumferential direction of the tire. The plurality of ribs, particularly ribs at opposite ends of the tread, are circumferentially continuous. However, tires mounted on floating wheels of vehicles, such s front wheels or trailer wheels, have a disadvantage that abrasion arises first on the ribs on the opposite ends at the early stage of abrasion as compared with ribs disposed at the central area of the tread this results in unbalanced abrasion.
Examples of the various types of unbalanced abrasion include edge-dropped abrasion in which the outside end portion at the opposite-side ribs is circumferentially nearly rectillinearly worn out, wavy abrasion in which the edge-dropped abrasion unevenly grows up on the circumference of the outside ribs to thereby make the outside ribs wavy on the circumference, shoulder-dropped abrasion in which the wavy abrasion further grows so that the wide area at each of the opposite-side portions of the tread is early worn out, and polygonal abrasion in which the wavy abrasion develops up to the ribs located on the central side over the main grooves so that the shoulder portion is shaped like a polygon on the circumference.
Various proposals have been proposed in order to prevent such unbalanced abrasion. Examples include a tire in which a large number of radially extending sipes are circumferentially disposed with respect to the ribs on the opposite sides of the tread. Also proposed is a tire in which the end portions on the opposite sides of the tread are cut into a round shape or a taper shape to form a truncated cone.
It is recognized that those conventional proposals have some beneficial effects, however, there is a problem in that, under the sever conditions where the ratio of the linear road portion is small while the ratio of the curved road portion is large, unbalanced abrasion is still apt to be generated. This wear is shoulder-dropped abrasion with wavy abrasion generated to a considerable extent so that the tire wears to drop its shoulder portion at the early stage of abrasion. Therefore the tread width in contact with the road surface is reduced, resulting in not only shortening tire lifetime against abrasion but degrading in stability of steering especially in the period of linear running.
The frequency of occurrence of such unbalanced abrasion is high especially in a half portion of the tire tread located at the outer side of a tire of a wheel when the wheel is mounted on a vehicle as its front wheel acting as a front idler or floating wheel. Although rotation of tires is sometimes carried out to cope with this problem, this work is troublesome and frequently ignored, and therefore tires are used mainly in the state where they are fixedly attached to an axle throughout tire life so that the problem of unbalanced abrasion becomes actualized.
As another example of a conventional tire is U.S. Pat. No. 3,411,559 (Re. 30,527) which discloses a tread pattern provided with two wide main grooves and several narrow subsidiary grooves for improving the attachment of tire to the road surface. However, it has been found that only the provision of those main grooves together with those subsidiary grooves cannot provide a sufficient effect to solve the problem of the foregoing various kinds of unbalanced abrasion.
As the result of various examinations, it has been confirmed that the tire which is proposed in Japanese Patent Application No. 2624780/1984 (corresponding to U.S. patent application Ser. No. 807,582, filed Dec. 11, 1985) uses circumferentially extending opposite-side ribs formed at opposite side ends of a tread of the tire and is sectioned into two portions by a narrow groove having such a width that the opposite walls of the narrow groove may come into contact with each other when the tire is loaded. This configuration is extremely effective to cope with the problem of unbalanced abrasion in the point that, of the two ribs divided by the narrow rib, shoulder dropped abrasion is generated only in the rib located on the outer side in the direction of the tire axis while no shoulder dropped abrasion is generated in the whole of the circumferentially extending rib located at the tread center portion. That is, the inventors of this application have appreciated such an advantage that the propagation of shoulder dropped abrasion to the tire-center-sided rib can be prevented by using the above-mentioned narrow groove.
However, it has also been found that there is a further problem in the reduction in lifetime against abrasion as well as in stability in rectilinear running, because at each of the opposite end sides of the tire, shoulder abrasion is generated in the tire-axially outer one of two ribs sectioned by the circumferentially extending narrow groove. The axial width of the tread contacting with the road surface becomes narrower.
As a result of further investigation, the inventors have found that the outside portion of the tread against a vehicle, when the tire mounted on the vehicle is especially mounted on the front wheel, is subjected to more severe input (distortion) from the road surface then the inside portion of the tread toward the vehicle. Thus, the inventors have attained the present invention.